There is much literature available concerning the production and uses of synthetic membranes. See, for example, Kesting, R. E., Synthetic Polymeric Membranes Structural Perspective, 2d Ed., John Wiley and Sons, Inc., New York, 1985, Porter, M. C., Ed. Handbook of Industrial Membrane Technology, Noyes Publications, 1990, Chapter 1, "Membranes and Their Preparation", (chapter by Heiner Strathmann), 1990, and D. R. Lloyd, ed., Materials Science of Synthetic Membranes, American Chemical Society, Washington, D.C., 1985.
As discussed in detail in those references and elsewhere, membranes prepared from synthetic polymers may be porous (i.e., void-containing) or dense, symmetric or asymmetric. Symmetric membranes exhibit essentially homogeneous morphology from one surface to the other whereas asymmetric membranes exhibit a morphology which varies continuously from one surface to the other. Each type of membrane is optimal for particular applications.
A very important general method for the production of membranes from various synthetic polymers is the so-called phase inversion process in which a single-phase polymer solution is subject to a condition under which a polymer rich phase is separated to form a membrane like structure. The phase inversion process is particularly useful for forming very fine pore symmetric membranes, as well as asymmetric membranes. Control over the conditions under which the phase separation occurs allows the pore structure to be tailored to particular needs. Asymmetric membranes, having a very finely porous or dense polymeric skin on an otherwise "open" membrane structure, offer high membrane selectivity coupled with high permeate transport rates.
One particular form of the phase inversion process is the so-called thermal phase inversion process developed by Castro (U.S. Pat. No. 4,247,498). In this process, a polymer solution is formed in a solvent at a temperature above the lower critical solution temperature, the resulting solution is then, e.g., cast onto a substrate or spun into a fiber under conditions in which the temperature of the solution is reduced to a point at which is formed a polymer rich phase from which the final membrane morphology is derived. The solution concentration, rate of cooling, and method of final quenching all influence the final morphology.
As a general rule, polymer solutions suitable for use in the thermal phase inversion process contain at least 20% polymer (see Chapter 10 of Materials Science of Synthetic Membranes, above, and temperatures have been below 200.degree. C.).
One particularly useful synthetic resin from which phase inversion membranes have not heretofore been made is polytetrafluoroethylene (PTFE). The absence of any known solvent for PTFE precluded any attempt in the direction of fabricating PTFE membranes by the phase inversion process. Thus, there are also no examples of true asymmetric membranes of PTFE. Other methods for fabricating symmetric membranes of PTFE, primarily so-called microporous PTFE, have been developed which are capable of creating PTFE membranes of relatively limited morphological scope. Notable among these are Gore-Tex microporous PTFE manufactured by W. L. Gore and Associates, Elkton, Md. (U.S. Pat. No. 3,664,915, U.S. Pat. No. 3,953,566, U.S. Pat. No. 3,962,153, 4,187,390). Gore-Tex exhibits a highly fibrillar morphology.
U.S. Pat. No. 4,248,924 discloses a porous asymmetric PTFE film wherein the asymmetry is produced by compression and a temperature gradient. The resultant morphology is highly fibrillated.
U.S. Pat. No. 4,863,604 discloses an asymmetric laminated sheeting structure in which each successive lamina exhibits a different pore size. This laminar structure is quite different from the structure of a typical, single layer asymmetric membrane produced by the phase inversion process in which pore size is observed to vary continuously from one surface to the other. Further, it is known in the art that PTFE is extremely difficult to form into laminates, and such laminates are highly susceptible to undesirable peeling and separation while in use. Because of the single layer structure of asymmetric membranes, they are not subject to peeling and separation of layers since there is only one, albeit inhomogeneous, layer.
U.S. Pat. No. 4,889,626 discloses an asymmetric tubular membrane made using PTFE and fluoropolymer alloys. The resulting membrane has a highly fibrillar morphology.
Other composite PTFE structures are known. R. West et al., Proc.-Inst. Environ., Sci., Performance of New Dual Asymmetric PTFE Membrane 37th (1991). The filter is characterized by having three layers, a thin inner layer of fine pores with layers above and below it having relatively larger pores.
F. Vego et al., Journal of Applied Polymer Science, Vol. 21, 3269-3290 (1977) disclose the preparation of asymmetric PTFE membranes using sintering of PTFE emulsions in the presence of salts. The porous supports formed were then deposited on a dense file of PTFE.
In order to fully realize the potential of the chemically inert, high temperature resistant PTFE in membrane applications, it is highly desirable to discover a phase inversion process for producing PTFE membranes, both symmetric and asymmetric in a single layer of pure PTFE.
Recently it has been discovered that perfluorinated cycloalkanes such as perfluorotetradecahydrophenanthrene and mixtures of oligomers formed therefrom can be employed at temperatures of ca. 300.degree. C. or higher to form solutions of PTFE. (U.S. Pat. No. 5,328,946). An attempt was therefore undertaken by applicants to fabricate PTFE membranes by the thermal phase inversion process, and in particular to fabricate asymmetric PTFE membranes characterized by a single continuous layer and a spherical particle morphology.